1. Field of the Invention
The present invention relates to a developing device for use in image forming apparatuses in the electrophotographic process and an image forming apparatus having the developing device.
2. Description of the Related Art
A developing device for use in an image forming apparatus in the electrophotographic process such as copying machines, printers, facsimiles, and multifunctional peripherals (MFPs) in combination thereof forms a toner image by flying toner conveyed by its developing roller to the surface of a photosensitive drum carrying an electrostatic latent image formed based on image data thereon. The image forming apparatus having such a developing device transfers the toner image formed on the photosensitive drum onto a recording medium such as paper. The toner image thus transferred is then fixed on the recording medium as heated in a fixing device. In this way, the image forming apparatus forms an image on the recording medium based on the image data.
Thus, the developing device installed in the image forming apparatus should convey the toner efficiently for example with its developing roller and charge its toner uniformly and reliably in forming a high-quality image.
Developing devices having a developing roller of aluminum that was surface-treated for example by plating or alumite treatment and those having a developing roller of stainless steel are known as conventional developing devices. An example of known developing device of which the charge on the toner is specified is a nonmagnetic mono-component developing device using a toner charged for example by corona charger, wherein at least the surface of the toner conveyer (developing roller) is made of a material containing a charge-controlling agent that is charged in the polarity same as that of the toner and the frictional charge on the toner and that on the material for the toner conveyer have a particular relationship (see JP-A No. 2-79062 and literature D1).
Often in conventional developing devices, more movable toner is supplied preferentially onto the photosensitive drum, leaving less movable toner deposited on the developing roller. Such a state is encountered frequently, in particular when a low-density image is printed, and continued printing for an extended period in the state results in charge up (overcharge) of the toner in the area close to the developing roller, for example by friction of the developing roller or the blade with the toner. Such partial charge up of the toner leads, for example, to deterioration in image density and image blurring, prohibiting printing of high-quality images over an extended period of time.
Specifically, for example, printing by using a toner having a volume-average diameter of approximately 6 μm leads to deposition of one to three toner particles superimposed on the developing roller, forming a toner thin layer having a thickness of approximately 6 to 18 μm. Repeated development leads to accumulation of highly charged small-diameter toner particles on the developing roller, and the toner present in toner surface layer that is more separated from the developing roller is supplied to the photosensitive drum preferentially. In such a case, the following disadvantages emerge.
The highly charged small-diameter toner, which has greater adhesive force to the developing roller, is not used for development, resulting in deterioration in image density. For use of such a highly charged toner in development, the difference in electric potential between the photosensitive drum and the developing roller should be widened for separation of the toner from the developing roller by interaction stronger than the adhesive force. However, increase in the difference in electrical potential between the photosensitive drum and the developing roller leads to electrical leakage between the photosensitive drum and the developing roller, disturbing the toner layer on the developing roller and thus, disrupting the image obtained.
Further in the case of a developing device in the touchdown mode, the toner travelling in the developing region occasionally remains deposited on the developing roller because of strong adhesive force in the developing device, even though it should be separated once from the developing roller. The toner remaining deposited on the developing roller is charged up to a greater amount, thus prohibiting printing of high-quality images even in the case of a developing device in the touchdown mode.
In literature D1, the charge on the toner deposited on the developing roller is specified in relationship with the frictional charge on the material for developing roller, but distribution of the charge of toner is not controlled at all. In such a case, the toner held on the developing roller, in particular the toner closer to the developing roller, is charged up. It may thus lead to insufficient image density and image blurring, prohibiting printing of high-quality images over an extended period of time.